1. Field of the Invention
This invention relates to an optical device, particularly but not exclusively for use as an optical amplifier, modulator or logic element.
Throughout this specification by interferometric coupled loop is meant a device having an optical cross-coupler having a first and a second pair of communication ports which splits an optical signal received at one port of given pair of ports to the two ports of the other pair of ports and an optical waveguide optically coupling the first pair of ports. In such a device, light entering one of the ports of the second pair of ports is split into two portions having relative intensities determined by the coupling ratio of the cross-coupler which portions exit respective ports of the first pair of ports to travel in opposite directions around the waveguide to re-enter the cross-coupler. The two portions are then each cross-coupled to the two ports of the second pair of ports where recombination takes place to provide two outputs. The term "optical" is intended to refer to that part of the electro-magnetic spectrum which is generally known as the visible region together with those parts of the infra-red and ultra-violet regions at each end of the visible region which are capable of being transmitted by dielectric optical waveguides such as optical fibres.
2. Related Prior Art
As is well known, a linear interferometric coupled loop with a 50:50 coupling ratio acts as a mirror in that a signal entering the cross-coupler at a port of the second pair of ports will exit entirely from that same port, none being output from the other port of the second pair. This is because the two signal portions travelling in opposite directions round the waveguide have no relative phase shift when they re-enter the coupler and so they each split in two portions on cross-coupling which destructively and constructively interfere at the second pair of ports. A paper entitled "Soliton logic elements for all-optical processing" by N. J. Doran, K. J. Blow and D. Wood Proc O-E Fibre, San Diego (1987) discloses an interferometric coupled loop which utilizes the instantaneous Kerr nonlinearity of the refractive index of silica to provide an output which is a function of the intensity of the light entering the input port by providing a coupler with a coupling ratio that is not 50:50. The two portions of the optical signal therefore have different intensities and so experience different instantaneous refractive indices throughout their passage around the loop. This gives rise to a phase shift proportional to the intensity and the distance propagated, referred to as self-phase modulation (SPM), different for the two portions. The result of the differently phase shifted signal portions arriving back at the coupler is that signals of different intensities are output from the two ports of the first pair of ports which can be used in a variety of applications including logic elements, optical amplifiers, modulators and the like.
There are several disadvantages associated with the use of this known device for pulsed signals. Because the effect responds rapidly (about 5 fs) to the varying local optical intensity the SPM will vary with the local intensity throughout the pulse. For fast, in the order of GHz, operations the pulses will not in general be square and so the variable SPM seriously degrades the characteristics of the device by producing incomplete switching and a poor on-off contrast ratio. As proposed in the above referenced paper, the efficiency can be improved in this pulse mode of operation if group-velocity dispersion effects are taken into account and the pulses are so shaped and are of sufficient power so as to propagate in or near the soliton regime. The Kerr coefficient in silica is small (n.sub.2 =3.2.times.10.sup.-16 cm.sup.2 W.sup.-1) which combined with the need for significant dispersion necessary to obtain soliton propogation requires a relatively long non-linear waveguide (from 1 to 4 m) and high power densities and appropriate pulse shaping. Also, when such known devices are used for logic operations between two or more signal streams, perhaps distantly originated, they must be coherent which is a very severe practical limitation.